This invention relates to an improved creosote wood preservative and a process for making the same by hydrogenating a conventional coal tar creosote.
Creosote has been used to preserve wood from decay and biological attack for many years. Creosote consists of liquid and solid aromatic hydrocarbons. Creosote is obtained from the high temperature carbonization of bituminous coal as a distillate boiling in the 200.degree.-400.degree.C. range. Although creosote is an excellent wood preservative, it has several problems. One problem is that creosote has a dark color which is imparted to wood treated with creosote. Another problem is that crystals form in the creosote when it is not kept hot. This crystal formation is a problem when creosote is transported or stored. Whenever a car or tank of creosote cools, crystals form and settle out and it is difficult to get all the settled crystals back into solution by reheating the creosote.
Creosote may also be used with pentachlorophenol as a wood preservative. Pentachlorophenol in a 5% solution with petroleum is not corrosive to metal equipment used in the wood-treating process. Creosote when used alone as a wood preservative is only slightly corrosive to the metal equipment used in the wood-treating process. However, when creosote and pentachlorophenol are used in a mixture, the ordinary steel or iron equipment used in the treating process is subjected to an intensive corrosion action. The prior art tries to overcome the problem of a corrosive pentachlorophenol, hereinafter referred to as penta, in creosote admixture by adding corrosion inhibitors. Not all corrosion inhibitors are effective on a penta in creosote admixture and those that are effective have a high cost.
The hydrogenation of hydrocarbon oils such as petroleum oils, coal extract and creosote is old in the art. The art has hydrogenated these hydrocarbon oils for many reasons and many of these reasons involved the lowering of the aromatic content of the hydrocarbon oils. Creosote has been hydrogenated in the past under severe conditions to produce motor fuel and jet fuel. A high temperature coal tar creosote has been hydrogenated to increase the toxicity of the high temperature creosote. Also, a wood preserving creosote complying with American Wood-Preservers' Association (A.W.P.A.) specifications has been hydrogenated at less than severe conditions to produce a beneficiated creosote. These conditions include a temperature of 395.degree.C., but generally a temperature above 300.degree.C., and a pressure of 225 atmospheres and 14,000 ft..sup.3 of hydrogen per barrel of creosote and a reaction time of 45 minutes and the presence of a molybdenum sulfide and aluminum chloride catalyst. The beneficiated creosote has a toxic value in excess of the original creosote. The temperature, pressure, time and hydrogen flow are coordinated so as not to induce the formation of a substantial percentage of nontoxic constituents as for instance chain structures, that would preclude enhanced toxic induction. Apparently, in this method of hydrogenation, any reduction of aromatic content of the creosote is to be avoided. Illustrative of the prior art pertinent to such hydrogenation of creosote are the following: U.S. Pat. No. 2,291,295 (Harvey), U.S. Pat. No. 2,291,330 (Harvey), and an article in Gas Eng 52, 15-17 (1935) by J. G. King.